(a) Field of the Invention
This invention relates to field of geophysical prospecting and more particularly, but not by way of limitation, to a method for generating a seismic vibrator sweep using frequency as a determinant for the vibrator force output applied to the subsurface of the earth.
(b) Discussion of Prior Art
In the oil and gas exploration industry, geophysical tools and techniques are commonly employed in order to delineate a subterranean structure for adequate hydrocarbon deposits. Many different techniques are used to generate a seismic signal.
In the early 1960's, seismic vibrators were developed and used in the field. A seismic vibrator is used as an energy source. A seismic vibrator in its simplest form is merely a heavy vehicle that has the ability to shake the ground at a predetermined range of frequencies. Normally in a range of 2 to 100 Hz. Using the seismic vibrator has the advantage of not being an impulsive energy source such as dynamite, which releases all of its energy at one time and can pose safety and environmental concerns. Instead, the vibrator imparts a signal into the subsurface of the earth over a longer period of time thus decreasing the energy level.
The imparted energy generated travels through the subsurface and reflects some of the energy from certain subsurface geological boundaries or layers. The reflected energy is then transmitted back to the earth's surface where it is recorded using an earth motion detector. The recorded data is processed to yield information about a location and physical properties of layers making up the location's subsurface.
The seismic vibrator generates a signal, also known as a sweep. The vibrator applies the sweep to the subsurface of the earth. Sweeps are sinusoidal vibrations and are typically in a range of 2 to 100 Hz. The vibrations last a duration of 2 to 20 seconds depending on the terrain, the subsurface lithology, economic constraints and physical capabilities of the vibrator. The sinusoidal sweep can be increased in frequency overtime, which is called an "upsweep". The upsweep is the signal used typically in modern seismic exploration. Also, the sinusoidal sweep can be decreased in frequency overtime, which is called a "downsweep". The end products of the vibrator sweep are waves that propagate through the earth to return clues about the subsurface.
Waves that are generated by a seismic vibrator can be classified into two categories. The first category contains high-frequency waves and normally in a range of 50 to 100 Hz. The higher frequency waves are responsible for much of the temporal resolution seen in the collected data. The second category includes the powerful low frequency waves that penetrate deep into the earth's surface. The low frequency waves are less than 50 Hz.
Both types of waves are collected to produce images of the subsurface that are then used to make predictions for the best drilling locations to tap hydrocarbon reserves.
The problem with the prior art seismic signals or constant force sweeps is the inability to separately manage the harmful low frequency signals from the less harmful higher frequency sweeps. An undesirable byproduct is to use a lower force throughout the entire sweep when conditions demand that the harmful low frequency waves be suppressed. The end result is a seismic energy source that has a continuously low force throughout its range of frequencies. The use of a continuously low force precludes the use of the vibrator's full energy potential for effective seismic exploration.
Therefore, a need exists for a method of separating low and high frequency force output or linking the force assimilated to the subsurface of the earth as a function of frequency. The subject invention as described herein provides such a method.